Remote control system



Aug. 6, 1957 Filed July 6, 1945 TIEG'E 7' [00,856

E. M. WILLIAMS ETAL REMOTE CONTROL SYSTEM 4 Sheets-Sheet 1 N/JJ/LE rmeair r m: 1770/24? I l l l l 6 1 5/052 Para/arm cmvrea;

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REMOTE CONTROL SYSTEM Filed July 6, 1945 4 Sheets-Sheet 3 INVEN TOR. [raw/w x-z Mal/i175 BY 547M a 6009') REMOTE CONTROL SYSTEM 4 Sheets-Sheet 4 Filed July 6, 1945 3/66 7147? Cal/TFO! a r I m m; W; W; w 2 W W W W 5 w a w 4 6 w J, |l|kl| T a a 4% n m a I H H WM n. u ww wa i Pr r a m2 MW 5 v a r :7 z a: n M w T llllllllll IL United States Patent ()1 REMOTE CONTROL SYSTEM Everard M. Williams, State College, Pa., and Edwin V. Cousy, Dayton, Ohio Application July 6, 1945, Serial No. 603,570

2 Claims. (Cl. 244-44) (Granted under Title 35, U. S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to us of any royalty thereon.

This invention relates to radar control of the course of a missile in flight, and more particularly to a means and a method for the directing of a missile toward an airborne target in order to intercept and destroy the target.

It has been suggested heretofore that radar may be used in directing a missile towards an airborne target by using some form of split beam radar on the missile, together with automatic means for restraining it in its flight to a course along the beam split axis. In conformity with this concept, the radar beam is directed to the target and the missile is caused to engage the target. One diflicnlty with this method is that when near the target the missile ismade to change the direction of its course sharply and erratically. This puts the automatic steering of the missile at very poor advantage. A system of missile control which avoids this difficulty has been devised herein.

The objects of the present invention comprises the provision of an improved means and method for causing missiles used in anti-aircraft fire and the like, to more precisely engage and destroy the targets toward which they are directed than has been possible in the past.

With the above and other objects in view that will be apparent to those who are informed in the radar art from the following description, suitable illustrative embodimentsof the present invention are shown in the accompanying drawings, wherein:

Fig. 1 is a diagrammatical presentation ofthe broad concept of the present invention;

Fig. 2 is a block diagram of a preferred circuit that is disposed at the control station and that emits signal that governs the flight of the missile that forms a part of the invention that is shown in Fig. 1;

Fig. 3 is a diagram of a split beam pattern of the signal that is emitted from the control station and that directs the flight of the missile that is shown in Fig. 1;

Fig. 4 is a primary block diagram and schematic circuit of the receiving station that is carried by the missile that is shown in Fig. 1;

Fig. 5 is a preferred block diagram and schematic circuit that is carried by the missile that is shown in Fig. '1;

Fig. 6 is a diagrammatical presentation of the first beam spreading means that forms a part of the invention that is shown in Fig. 1; and

Fig. 7 is a diagrammatic presentation ,of a double beam radar antenna.

In Fig. 1 of the accompanying drawings, a target radar 1 is beamed on a target 2 by having its beam 5 trained thereon. The target radar 1 is a radar system for detecting and following a target and preferably fully automatic in its tracking of the target 2 by means of the operation of its antenna 62 but hand-operated tracking may be used if desired. A missile radar 3 is shown beside the target Patented Aug. 6, 1957 radar 1. Missile radar 3 is a radar system which is used to direct a missile 4 to the target 2. The missile radar 3 is spaced laterally from the target radar 1 sutficiently to enable a missile 4 to be maintained clear of the beam 5 from the target radar 1. The missile 4 in its flight is designed to travel along or ride the beam 6 from a radar antenna 11 of the missile radar 3.

A receiving station 9 mounted in missile 4 receives signals from missile radar 3 and causes missile 4 to be directed along beam 6, by means which will be explained hereinafter. The elements of motion of target 2 as detected by target radar 1 in tracking target 2 are transferred to a known form of predictor 7 in which the future course of target 2, as derived from its past observed motion, is computed. This future course information is transferred to a beam rider control 8, in which a course for the missile 4 is computed. The future course information transferred from predictor 7 to beam rider control 8 may be in any of the commonly known forms such as a number of electrical potentials corresponding to the elements of the predicted motion of the target or several gear settings having similar import. The future course information may be transferred by means of mechanical means, such as a number of gears or rods which assume a position corresponding to the elements of the predicted future motion of the target. Beam rider control 8 may be any of the commonly known systems which can utilize the type of target course information transferred to it from predictor 7 and can add thereto the predetermined factors of speed of the missile 4, time of starting, Windage and other pertinent factors required for computing a course for missile 4. Beam rider control 8 may also include means commonly known in the art such as servo motor controls to which the computed missile course information is supplied and which aim missile radar antenna -11 in azimuth and elevation thereby aiming the radar beam 6 from the missile radar 3 along the computed course so that the missile 4, in riding the beam 6, will cross the predicted target course, and so that the missile 4 will hit the target 2 at the intersection of the target course and the beam 6. In our device the missile 4 is directed at the initiation of its flight directly to the point at which it will make predicted contact with the target 2. Subsequent variations in the control of the flight of the missile 4 are-only in the nature of minor variations of its course due to corrections in the predicted time and point of contact. The course of the missile 4 approaches a straight line and hence the automatic steering of the missile 4 is put at best advantage by operation of the present invention.

'Target radar 1, predictor 7 and beam rider control 8 are similar to the commonly known anti-aircraft fire control radar system such as the S. C. R. 584 detailed description of which is found in War Department Technical Manuals TM 11-1524 of March 1944 and July 1946. The difference is that beam rider control 8 controls the aiming of antenna 11 instead of an anti-aircraft gun.

A primary means of missile control whereby the missile 4 is held to a course on the beam 6 is shown in Figs. 2, 3, and 4 of the drawings. The components of the missile radar 3 that are shown in Fig. 2 comprise a motor 12 that is coupled mechanically, as indicated by a dash line, to a pulse generator 13 that feeds a divided output to a receiver 63 and to a modulator 14. The output of the modulator '14 applies pulses through a transmitter 10 and a T-R box 64 to an antenna 11. The TR box 64 also applies radar echoes received by antenna 11 to the receiver 63. The beam 6 that is radiated from the missile radar antenna 11 is of the normal split beam type and has an axis 16, as shown in Fig. 3 of the drawings. In order to radiate such anormal split beam, the dish 17, part of which causes an instantaneous radiation pattern 15, the pattern of which is shown in Fig. 3, to revolve continually about the beam axis 16, which is off the axis of symmetry 22 of the pattern 15. After half a revolution, the radiation pattern is oriented as at 15. One precession of the dish 17 corresponds to one'rotation of the instantaneous radiation pattern. In the apparatus of the present invention the beam polarization does not rotate but remains invariable in direction in usual manner. The axis 16 of the beam 6 is directed by the turning of antenna 11, which is effected by the beam rider control 8. The precession of the dish 17 is effected by a mechanical connection to the motor 12 in usual manner, as indicated in Fig. 2 by a dash line connecting the motor 12 to the radar dish 17. The pulse that terminates the radar pulse that is transmitted from the antenna 11 is generated in the pulse generator 13. The repetition rate of the pulses from pulse generator 13 is arranged to change four times during one precession of the dish 17. Thusthe different positions of the instantaneous radiation pattern 15 are distinguished by the contemporaneous pulse repetition rate.

The movement of the missile radar pattern 15 about the axis 16 is made more explicit by reference to Fig. 3. In Fig. 3 a circle 23 is disposed in a plane that is perpendicular to the beam axis 16 and is centered on a point lying in that axis. Circle 23 is divided into quadrants 18, 19, 20 and 21. The axis of symmetry 22 of the instantaneous radiation pattern 15 passes through these quadrants 18, 19, 20 and 21 as it rotates. As long as the axis of symmetry 22 lies in the quadrant 18, the pulse repetition rate is at one rate. As soon as the axis 22 passes to the next quadrant 19 the pulse repetition rate changes to a difierent rate and so on among the quadrants 18, 19, 20 and 21.

The missile 4 is equipped with a gyrostabilizer 28, shown in Fig. 1, which prevents the missile 4 from turning about its longitudinal axis. The missile 4 is placed initially with its axis in the axis 16 of the radar beam so that the rotation of the missile 4 about the axis 16 of the beam 6 also is prevented.

The circuit of a receiving station 9 in the missile 4, is shown in Fig. 4 and comprises a non-polarized antenna 24 that intercepts signals emitted from the missile radar 3, a missile receiver 25 that receives intercepted signal from the antenna 24, and a plurality of balanced detectors 26 and 27, that are transformer coupled with the missile receiver 25.

The missile receiver 25 receives pulses of the radar beam 6. Within the missile receiver 25 the pulses of the radar beam 6 are detected, lengthened, and an approximate sinusoidal output is generated which has the frequency of the corresponding pulse repetition rate and an intensity that is proportioned to the received signal strength. The output from the missile receiver 25 is passed to the balanced detectors 26 and 27 in parallel. If the missile 4 lies on the axis 16 of the radar beam 6, pulses of the four repetition rates in the quadrants 18, 19, 20, and 21, will be received in turn and all equally. If, however, the missile 4 drifts off of the axis 16 of the radar beam 6, as into the quadrant 18, pulses of the rate corresponding to those in the quadrant 18 will be received in greater intensity than those of the frequency corresponding to the opposite quadrant 20.

Two filters 29 and 30 are disposed in the balanced detector 26 and are tuned respectively to the two frequencies, or pulse repetition rates, which characterize quadrants 18 and 20. The filters serve to isolate each of the signals from quadrants 18 and 20. The signals so isolated are detected in diodes 31 and 32 and the rectified output appears across balanced resistors 33 and 34, respectively. The direct voltage across resistor terminals 35 and 36 of the resistors 33 and 34, respectively, has an average magnitude that is proportional to the average difierence between the received intensities of the two frequency characterized pulse groups corresponding to quadrants 18 and 20 and has a polarity corresponding to whichever pulse group is the more intense. The average voltage output from the balanced detector 26 is applied to a usual form of up-down control 46 for changing the elevational course of the missile 4. In the present example, which assumes the signal corresponding to quadrant 18 to be stronger, control 46 will serve to direct the missile 4 away from the quadrant 18 of the circle 23 and along the axis 16.

The balanced detector 27 is substantially a duplication of the balanced detector 26 and comprises a pair of filters 37 and 38, respectively, which are tuned to the pulse repetition rates of quadrants 19 and 21, and which apply signals to diode detectors 39 and 40. The outputs of the detectors 39 and 40 are applied across balanced resistors 41 and 42, the terminals 43 and 44 of which are fed to a right-left or azimuth control 45. In similar manner to the operation described above, if the missile 4 strays into the quadrant 21 of the circle 23, the mechanism in the balanced detector 27 directs the course of the missile 4 back to the axis of the radar beam 6.

The flight of missile 4 is monitored in receiver 63 of missile radar 3 in the manner well known in the radar art.

A preferred means of missile control, whereby missile 4 is held to a course on beam 6, is illustrated by further reference to Fig. 2 and to Fig. 5 of the drawings. The system there illustrated is simpler than the method that has been described above. In our preferred means separate designating pulse repetition rates for the four quadrants 18, 19, 20, 21 are not required and the pulse rate of missile radar 3 is now constant. In our preferred system of missile control the missile radar 3 conforms to that used in the system that is shown in Fig. 2, with a normal pulse generator replacing the pulse generator 13 therein. The normal pulse generator has a single steady pulse repetition rate. However, once during each precession of dish 17, the normal pulse generator initiates a quadrant 18 of Fig. 3.

For the purpose of explaining our preferred means of missile control quadrants 18 and 19 are taken as upper quadrants; quadrants 20 and 21 are taken as lower quadrants; quadrants 19 and 20 are taken as right hand quadrants; and quadrants 21 and 18 are taken as left hand quadrants. The rotation of radiation pattern 15 is through the quadrants in the order 18, 19, 20, 21.

The receiving system which is provided for use in the preferred means of missile control is shown in Fig. 5. This receiving system is disposed within the missile 4. The antenna 24' and the receiver 25 function to receive the radar beam signal from missile radar 3 with the marker pulse above described. The audio output from receiver 25' consists of a train of waves at the frequency of the normal pulse repetition rate with a train of superposed pulses at the marker pulse rate.

This marker signal is isolated from the rest of the audio signal, in a marker pulse detector 47, preferably on the basis of amplitude discrimination, or by other means. The output of marker pulse detector 47, which is a positive pulse at the marker pulse rate, is passed to an up-down flip-flop circuit 49 and through delay network 48 to a right-left flip-flop circuit 50. p

A flip-flop circuit is a double relaxation oscillator of quasi-self-determined cycle, in which a first tube-passes plate current and a second tube is biased below cutoff during the first half of each cycle, and then the first tube is biased below cut-ofi and the second tube passes plate current during the succeeding half of the cycle. The oscillation is initiated by a positive firing pulse applied to the grid of the first tube. In the present device this firing pulse is the marker pulse from marker pulse detector 47. It is characteristic of a flip-flop circuit that, if its natural period is made slightly longer than the period of the firing pulse, the firing pulse will take control and impress its period -on the flip-flop circuit. Flip-flop circuits 49 and 50 operate in this manner.

The output of fiip-flop circuit 49 consists of two square waves of voltage, one from each of its two tubes. These two waves are at the marker pulse rate and are 180 degrees out of phase with each other so that there is positive voltage in one wave when there is zero voltage in the other, and vice versa.

The marker signal which passes from marker pulse detector 47 to flip-flop circuit 50 is delayed in delay network 48 by one quarter cycle of the marker pulse rate. The construction and operation of flip-flop circuit 50 is identical with that of flip-flop circuit 49. Owing to the delay introduced by network 48 the output of flip-flop circuit 50 which is also two square waves at 180 degrees to each other, is delayed 90 degrees behind the corresponding output of flip-flop circuit 49.

It can thus be seen that the outputfrom up-down flipflop circuit 49 is two square waves spaced 180 degrees apart, the first occuring simultaneously with the reception of the synchronizing marker pulse occurring at the instant that radiation pattern enters quadrant 18 of Fig. 3 and lasting until the occurrence of the second square wave which is when radiation pattern 15 enters quadrant 20. The second square wave lasts until the flip-flop circuit is tripped by the synchronizing marker pulse again which is when radiation pattern 15 enters quadrant 18 again. Similarly, since synchronizing marker pulses are delivered to right-left flip-flop circuit 90 degrees later, its output will be a first square wave While radiation pattern 15 traverses sectors 19 and 20, and a second square wave while radiation pattern 15 traverses sectors 21 and 18.

The full output from receiver 25' consisting principally of audio signal at the pulse repetition rate is passed to four tubes in parallel. These tubes are denominated up gate 51, down gate 52, right gate '53, and left gate 54. These tubes are of the type which contain two control grids, and the audio signal is applied to the first control grid in each tube. The square wave outputs from flipflop circuits 49 and 50 are applied to the second control grids of the four gating tubes in the following succession: output of first tube of flip-flop circuit 49 to up gate 51, output of second tube of flip-flop circuit 49 to down gate 52, output of first tube of flip-flop 50 to right gate 53, and output of second tube of flip-flop 50 to left gate 54. The tubes are all normally biased at cut-off but will pass plate current upon the application of the square wave to the respective second control grids. The tubes are thus arranged to act as gates passing a signal only upon receipt of a gating signal from the appropriate flip-flop circuit.

It will be seen that the result of the operation of tubes 51, 52, 53, 54 is to pass the audio signal resulting from the normal pulse transmission received from missile radar 3 in the following cycle: Up gate 51 passes signal resulting from normal pulse transmission, or normal signal, during the first half of the marker cycle, that is, normal signal transmitted when radiation pattern 15 is in the upper quadrants 18 and 19. Down gate 52 passes normal signal during the second half of the marker cycle, that is, normal signal transmitted when the radiation pattern 15 is in the lower quadrants and 21. Right gate 53 passes normal signal during the second and third quarters of the marker cycle,.that is, normal signal transmitted when radiation pattern 15 is in the right quadrants 19- and 20. Left gate 54 passes normal signal during the third and fourth quarters of the marker cycle, that is, normal signal transvmitted .when radiation pattern 15 is in the left quadrants Z1 and 18.

The outputs of gating tubes 51 and 52 are fed to the two sides of a balanced detector 55. It is the property of a balanced detector that the output thereof is a direct current whose polarity is determined by whichever of its two input signals is the more intense, and whose intensity is proportional to the difference between the intensities of the .two input signals. Output of balanced detector 51 is applied to, .and activates; up-down control 46'. Updown control 4.6 is identical in construction and operation with up-down control 46 previously described.

Similarly the outputs of gating tubes 53 and 54 are fed to the twosides of balanced detector-56, which is identical in structure andoperation with balanced detector 55. The output thereof is applied to, and activates, right-left control 45, which is identical in construction and operation with right-left control .45: previously described.

All circuits shown in Fig. 5 are understood to have return through ground.

The manner in which our preferred means of missile control operates to hold the course of missile 4 within beam .6 will nowbe explained. It will be seen that so long .as missile 4 stays on beam axis 16 the normal pulse signals received will .be equal as radiation pattern 15 rotates through all four quadrants 18, 19, 20, 21, and the outputs of balanced detectors 55 and 56 are null. It will be seen that the-combined action of flip-flop 49, up gate 51, the balanced detector 55 is to integrate the normal signal received when radiation pattern 15 is in quadrants 18 and 19, that is, when .it is above the beam axis 16. Similarly the combined action of flip-flop 49, down gate 52 and balanced detector 55 is to integrate the normal signal received when. radiation pattern 15 is in quadrants 20 and 21, that is, -when it is below the beam axis 16. It now follows that, if missile 4 strays above beam axis 16, the signal received when radiation pattern 15 is above the beamaxis .16 is :stronger than that received when radiation pattern 15 is below the beam axis 16. Accord ingly balanced detector 56 develops an output whose polarity isin accordance with the stronger signal to return missile 4 left toza position on axis 16. Corrective action in the eventof-missile 4 straying to the left is analogous.

At the start -of the flight of the missile 4, the missile radar beam 6 is very sharp and the signals are very strong. In order to weaken .the forcefulness of the controls, which may cause overshooting, and to extend the cross sectional area over which the beam is operative, it is desirable to have a broader beam at the start of flight than at the end A first form .of beam broadening is shown in Fig. 6. The equipment there shown comprises a double reflector antenna for the missile radar 3. The double reflector antenna for the missile radar 3 comprises a radiator 57, a small parabolic reflector 58, and a large parabolic reflector 59. The reflector 59 is hollowed out to receive the reflector 58. Both reflectors 58 and 59 have the same focal length. At the start of the flight of the missile 4, the reflector 59 is moved away from the radiator 57 so that no rays from the radiator 57 reach the reflector 59. On page 838 of The Radio Engineers Handbook by F. E. Terman (McGraw-Hill Book Co., Inc., published 1943) is found, The width of the main lobe between nulls, assuming that the radiation from the exciting antenna is non-directional, is given approximately by the equation Where D/A is the mouth diameter in wavelengths.

Beam width between nulls in degrees:

flight of the missile 4, the reflector 59 is moved up to the dash line position 59' so thatit reflects rays from the radiator 57. The beam width 61 then formed is narrow due to the increased reflector mouth diameter. It is to be understood that when reflector 58 and reflector 59 are used with missile radar 3 in place of reflector 17, they are precessed in the same manner as described above for reflector 17. However, the angle of precession may be increased with the broader radiation pattern obtained from reflector 58 in order to reduce the central overlap area and then the angle of precession may be reduced upon the utilization of reflector 59 which gives the narrower radiation pattern.

A second system for beam broadening is shown in Fig. 7, wherein a double beam radar antenna 65 is disclosed. Double beam radar antennua 65 comprises two distinct radiators 66 and 67. Radiator 66 is excited at a higher frequency than radiator 67 Radiator 66 is mounted at the focus of the processing parabolic reflector 68 in a normal manner and the array composed of radiator 66 and reflector 68 produces a sharp beam on the higher frequency. Radiator 67 is mounted somewhat displaced from the exact focus of reflector 68 due to the presence of radiator 66. Due to the fact thatthe radio beam reflected from a parabolic reflector of given dimensions is broader the lower the frequency of the radio waves the beam produced by the array composed of radiator 67 and reflector 68 is broader than the beam on the higher frequency from the array composed of reflector 68 and radiator 66. Stated another way, reference to the formula shown in the preceding paragraph shows that for a given array, as the frequency increases, D the mouth diameter decreases and therefore the beam width will be increased. Additional means well known in the art may be used to make the lower frequency beam from radiator 67 as broad as desired.

The double beam antenna 65 is used in place of the conventional antenna 11 in a missile radar similar to that shown in Fig. 2 with certain modifications. In Fig. 2, if antenna 11 is replaced by antenna 65, the transmitter 10, receiver 63, and T-R box 64 may be considered as operating together with radiator 66 on the higher of the two radar frequencies. In exactly similar fashion a duplicate transmitter, receiver, and T-R box are provided to operate with radiator 67 on the lower of the two radar frequencies. The rest of the radar system may be common to the two frequencies. Reflector 68 is precessed in a similar fashion as was reflector 17. However, the angle of precession may be increased somewhat with the broader radiation pattern obtained when radiator 67 is used in order to reduce the central overlap area and the angle of precession may thus be reduced when the narrow radiation pattern is produced by the use of radiator 66.

The form of missile receiving station 9 used with the double beam antenna 65 is similar to that shown in either Fig. 4 or Fig. 5 except that missile receiver 25 (in Fig. 4) or 25' (in Fig. 5) is replaced by a double frequency receiver capable of receiving both frequencies from the double beam antenna. A clock work mechanism in the double frequency receiver is set at the start of the missile flight so that the double frequency receiver is responsive only to the lower frequency on the broad beam during the early part of the missile flight but at a predetermined time later in the flight the double frequency receiver is made responsive only to the higher frequency on the sharp beam.

It is noted that the particular adaptation, arrangements, circuits and individual components that have been shown and described herein have been submitted for the purposes of illustrating and describing suitable representative embodiments of the present invention and that similarly operating modifications, substitutions, and changes may be made therein without departing from the scope of the present invention as defined by the appended claims.

What we claim is:

l. A system for controlling the flight of a missile comprising means for generating pulse modulated radio frequency energy, means for distinctively modulating said energy at repetitive intervals, means for directionally radiating said distinctively modulated energy, means for revolving said directionally radiated energy to form a solid conical radiation pattern, the period of a cycle of said revolution equalling said distinctive modulation interval, means for receiving said radiated energy at said missile, means for filtering said distinctive modulation from a portion of said received energy, means for synchronizing the generation of a first two sequential pulses each having a duration of half said cycle with a portion of said filtered modulation, means for delaying the phase of another portion of said filtered modulation degrees, means for synchronizing the generating of a second two sequential pulses each of half said cycle duration with said 90 degree phase delayed modulation, means for controlling the flow of the remaining portion of said received energy during the intervals of said sequential pulses, means for differentially combining the received energy flow during said first two pulse intervals, means for differentially combining the received energy flow during said second two pulse intervals, and means for controlling the missile flight along the axis of the cone of radiation in accordance with the resultants of said differential combinations.

2. A radio system for controlling the flight of a missile along the axis of revolution of a beam characterized by being modulated by a periodic pulse once for every revolution and revolved about an axis to form a solid conical radiation pattern, comprising means to receive said radiation, a filter to separate said periodic pulse from a portion of said received radiation, first relaxation oscillator means to generate a first two sequential pulses each having a duration of half said cycle of revolution, means to apply a portion of said filter output to said first relaxation oscillator means to synchronize its sequential pulse generation, a 90 degree phase delay network to delay the remaining portion of said filter output, second relaxation oscillator means to generate a second two sequential pulses each having a duration of half said cycle of revolution, means to apply a portion of said phase delay network output to said second relaxation oscillator means to synchronize its sequential pulse generation, first, second, third and fourth normally closed electronic switches, the remaining portion of said received radiation being applied to said switches simultaneously, one of said first two sequential pulses being applied to open said first electronic switch, the other of said first two sequential pulses being applied to open said second electronic switch, one of said second two sequential pulses being applied to open said third electronic switch, the other of said second two sequential pulses being applied to open said fourth electronic switch, means to differentially combine the output from said first and second electronic swtiches, means to diiferetially combine the output from said third and fourth electronic switches and means responsive to the resultants of said differential combinations to control the flight of said missile along the axis of said conical radiation pattern.

References Cited in the file of this patent UNITED STATES PATENTS 1,931,980 Clavier Oct.'24, 1933 2,082,347 Lieb et al June 1, 1937 2,165,800 Koch July 11, 1939 2,176,469 Moueix Oct. 17, 1939 2,397,088 Clay Mar. 26, 1946 2,404,942 Bedford July 30,1946 2,451,917 Chilowsky Oct. 19, 1948 (Other references on following page) 9 UNITED STATES PATENTS Becker Oct. 26, 1948 Kolding Jan. 4, 1949 Jenks July 19, 1949 Chu Aug. 9, 1949 5 Agins et a1 June 19, 1951 10 Deloraine June 26, 1951 Homrighous Dec. 25, 1951 Haller Apr. 29, 1952 Bedford et a1. Nov. 4, 1952 FOREIGN PATENTS Great Britain Aug. 16, 1940 

